A user equipment (UE) typically conducts wireless communications by transmitting and receiving electromagnetic signals via one or more antennas. Antennas are transducers for converting energy between electronic signals processed internally by the UE, and electromagnetic signals which propagate through a transport medium (such as the air). Such signals typically include a data component which contains information being communicated, and a carrier component which is used to modulate the data component and determines the centre frequency of the signal. Electrical signals applied to an antenna by a UE cause corresponding electromagnetic signals to be transmitted by the antenna. Likewise, electromagnetic signals received at the antenna cause the generation of corresponding electrical signals that can then be processed by UE circuitry (including demodulation of the signals to isolate data components from carrier components).
The efficiency of the power converted by the antenna depends on the impedance matching at the interface between the antenna and the UE circuitry (also known as the feed-point). The impedance of the feed-point is in turn influenced by the physical properties of the antenna. For example, a dipole antenna is best served to transmit and receive electromagnetic signals having a wavelength of twice (or close to twice) the length of the antenna conductor. This is because a standing half-wave is formed along the length of a dipole antenna. The frequency of an electromagnetic signal corresponding to such a wavelength is termed the antenna's natural resonant frequency. For a monopole antenna, the natural resonant frequency is the frequency of an electromagnetic waveform having a wavelength four times for close to four times) the length of the antenna.
The feed-point impedance experienced by a signal oscillating at the natural resonant frequency of an antenna is purely resistive, and hence provides for an efficient transfer of power between the antenna and the UE circuitry. However, for signals oscillating at frequencies that deviate from the natural resonant frequency of the antenna, the experienced feed-point impedance becomes increasingly reactive, resulting in a reduction in the power conversion efficiency. At such frequencies, converted signals may be too weak to be reliably isolated from general noise, resulting in poor reliability communications.
The rate at which the power conversion efficiency decreases as signal frequencies deviate away from the natural resonant frequency of the antenna is determined by further physical properties of the antenna. For a given frequency at a fixed deviation from the natural resonant frequency of the antenna, an antenna with a larger diameter conductor provides a feed-point impedance that is less reactive than an antenna with a smaller diameter conductor. Hence, antennas with larger diameter conductors provide a wider useful bandwidth in which energy can be reasonably efficiently converted.
Modern UEs conduct communications at frequencies in the multiple hundreds of megahertz or low gigahertz. To transmit or receive such signals with to naturally resonant antenna would require an antenna that is larger than would be comfortably portable. In order to maintain the portability of modern UEs, much smaller antennas are used. Such antennas are forced to transmit and receive signals at frequencies that are far away from the antennas natural resonant frequency. At such frequencies, the feed point impedance is almost entirely reactive and the power conversion efficiency is very low. In order to enable communications under such conditions, an electrical load (also known as a matching network) can be used to alter the resonant frequency of the antenna, as shown in FIG. 1.
At the desired communication frequency, antenna 100 provides a feed-point impedance at interface 102 that is largely reactive. In order to enable effective communications at the desired communication frequency, electrical load 104 is introduced. The impedance of electrical load 104 is selected to cancel the reactive feed-point impedance of antenna 100 at the desired communication frequency, thereby making the feed-point impedance entirely resistive at that frequency. This has the effect of tuning antenna 100 to have its resonant frequency at the desired communication frequency. Typically, this is achieved by selecting an electrical load of an equal but opposite reactance. In the case described above, where the communication frequency is much lower than the natural resonant frequency of the antenna, the feed-point impedance at the desired communication frequency will be capacitive. Hence, a corresponding inductive electrical load can be selected to cancel out the net reactance.
Recent developments in communications protocols, satellite positioning and other radio access technologies are putting further strain on antenna design constraints. For example, multiple-input multiple-output (MIMO; also known as diversity) schemes require the use of multiple antennas simultaneously, which further limits the space available to each one, and may provide differing dimensional constraints because the antennas require orthogonal orientation. Also, carrier aggregation schemes often require further antennas, each configured to conduct communications at different frequencies, and/or require the use of wider bandwidths, which results in further strain on the dimensional constraints.
Hence, it would be desirable to provide improved measures for tuning UE antennas.